Apparatus and methods for melting glass compositions for glass laser rods

ABSTRACT

Methods of melting and refining molten glass for the production of glass laser rods, the glass having a relatively high lithia content of at least about 8 mole percent, the glass being melted in a platinum lined furnace having a roof, sidewalls and a bottom having a discharge opening and a plunger adapted to open and close the opening. The method includes the steps of (1) charging into the furnace by means of a charge opening and a charge tube raw glass batch materials in the form of oxides, (2) melting the batch materials to form molten glass and refining the same, (3) providing an atmosphere of inert gas above the molten glass by a flow of gas into the atmosphere, and using the inert gas to purge any air from the furnace atmosphere during the charging of the raw batch materials, (4) stirring the molten glass with stirrers having platinum surfaces on the portion thereof exposed to the molten glass furnace atmosphere, (5) sealing the charge tube, the stirrers and the plunger with molten glass seals to prevent air from coming into the furnace atmosphere and to prevent condensation of platinum vapors to thereby eliminate deposits of platinum containing material in the molten glass which could cause platinum inclusions, and (6) charging the molten refined glass through the discharge opening for the production of high quality glass laser rods having a minimum of platinum inclusions.

United States Patent Chapman et al.

154] APPARATUS AND METHODS FOR MELTING GLASS COMPOSITIONS FOR GLASSLASER RODS [72] Inventors: George C. Chapman; James T. Le Sueur,

both of Toledo, Ohio [73] Assignee: Owens-Illinois, Inc.

[22] Filed: Nov. 17, 1969 [21] Appl. No.: 877,076

[52] US. Cl ..65/32, 65/129, 65/134,

[51] Int. Cl. ..C03b 5/18 [58] Field of Search ..65/32, 126,129,134,157,

[56] References Cited UNITED STATES PATENTS 2,113,956 4/1938 Wadsworth..65/330 X 2,155,131 4/1939 Hanlein ..65/157 3,209,641 10/1965 Upton65/335 X 3,343,982 9/1967 Maxwell et al.... ..65/32 X 3,471,409 10/1969Lee et al. ..106/52 X Primary Examiner-S. Leon Bashore AssistantExaminer-Robert L. Lindsay, Jr.

Attorney-Richard D. Heberling and E. J. Holler [151 3,656,924 [451 Apr.18, 1972 [5 7] ABSTRACT Methods of melting and refining molten glass forthe production of glass laser rods, the glass having a relatively highlithia content of at least about 8 mole percent, the glass being meltedin a platinum lined furnace having a roof, sidewalls and a bottom havinga discharge opening and a plunger adapted to open and close the opening.The method includes the steps of (1) charging into the furnace by meansof a charge opening and a charge tube raw glass batch materials in theform of oxides, (2) melting the batch materials to form molten glass andrefining the same, (3) providing an atmosphere of inert gas above themolten glass by a flow of gas into the atmosphere, and using the inertgas to purge any air from the furnace atmosphere during the charging ofthe raw batch materials, (4) stirring the molten glass with stirrershaving platinum surfaces on the portion thereof exposed to the moltenglass furnace atmosphere, (5) sealing the charge tube, the stirrers andthe plunger with molten glass seals to prevent air from coming into thefurnace atmosphere and to prevent condensation of platinum vapors tothereby eliminate deposits of platinum containing material in the moltenglass which could cause platinum inclusions, and (6) charging the moltenrefined glass through the discharge opening for the production of highquality glass laser rods having a minimum of platinum inclu- 810118.

17 Claims, 2 Drawing Figures kits APPARATUS'AND METHODS FOR MELTINGGLASS COMPOSITIONS FOR GLASS LASER RODS THE INVENTION The presentinvention relates to apparatus and methods for melting and refiningnormally highly corrosive lithium silicate glass compositions for theproduction of high quality glass laser rods with a minimum ofundesirable platinum inclusions.

In the past, it has been very difficult to make high quality glass laserrods with a minimum of platinum inclusions. The molten glass iscorrosive and even in platinum lined furnaces or melting tanks theresultant glass laser rods suffered from the drawback of havingundesirable platinum inclusions. It has been particularly difficult tomake high quality glass laser rods without an undesirable amount ofplatinum inclusions, particularly those glasses that have a high lithiumcontent in the neighborhood of about 15 to 35 mole percent based on thetotal glass composition.

It is an object of the present invention to provide an apparatus formelting and refining a lithium silicate glass composition having arelatively high amount of Li in the range of about 8 or up to as high asabout 35 or 40 mole percent to produce high quality glass laser rodstherefrom with a minimum of platinum inclusions.

It is an object of the present invention to provide methods of meltingand refining molten glass having a high lithia content of at least aboutmole percent based on the total glass composition.

It is an object of the present invention to provide an apparatus andmethod for melting and refining molten glass for the production of glasslaser rods, the apparatus including a furnace or melting chamber with aroof, sidewalls and a bottom, a charge opening in the roof, means forintroducing inert gas for the furnace atmosphere over the molten glassincluding an outlet conduit, and means for purging the furnace duringthe charging including an outlet conduit and a solenoid valve locatedtherein for evacuating air through the charge opening during the chargeoperation.

These and other objects will be apparent from the specification thatfollows, the appended claims, and the drawings in which:

FIG. 1 is a fragmentary front elevational view, partly in section, of afurnace for melting and refining molten glass for the production ofglass laser rods; and

FIG. 2 is a fragmentary front elevational view of a portion of thefurnace shown in FIG. 1 with the charge tube in position for chargingraw batch materials into the furnace.

The present invention provides an apparatus for melting and refining alithium silicate glass composition having a relatively high amount oflithia in the range of about 8 or 10 to 35 or 40 mole percent, theapparatus comprising a furnace for melting and refining the moltenglass, a roof, a platinum liner for the inside walls of the furnace, astirrer preferably made of platinum for stirring the molten glass, meansfor introducing an inert gas such as nitrogen for the furnaceatmosphere, means for withdrawing the gas from the interior of thefurnace so as to continually purge the same, means for withdrawing themolten glass within the furnace including a discharge opening and aplunger for the opening and closing of the discharge opening, means forcharging raw batch materials into the furnace including a charge openingand a charge tube adapted to fit therewithin, and means for sealing thecharge means, the stirring means, the plunger, and the juncture betweenthe sidewalls and the roof, to eliminate the condensation of platinumvapors which could cause platinum inclusions in the molten glass andalso to eliminate the entrance of air into the furnace atmosphere whichcould cause platinum inclusions in the resultant glass laser rod. Thefurnace or melting chamber is preferably heated electrically as is wellknown in the art and is housed within refractory sidewalls and a bottommember.

The present invention is also directed to methods of melting andrefining molten glass for the production of glass laser rods.

The methods include (I) charging into a platinum lined furnace having aroof, sidewalls and a bottom with a discharge opening and a plungeradapted to open and close the opening, certain raw glass-forming batchmaterials in a nonvolatile form, (2) melting the nonvolatile batchmaterials to form molten glass and refining the same in a furnace, (3)providing a nitrogen gas or other inert gas atmosphere above the moltenglass during the charging step and during the melting and refiningsteps, (4) stirring the molten glass with platinum stirrers, (5) sealingthe charge opening, the stirrers, the juncture between the roof andsidewalls, and the plunger with molten glass seals to prevent thedeposition of platinum vapors and to prevent the entrance of air intothe furnace to thereby eliminate undesirable deposits of platinumcontaining material which could cause platinum inclusions in theresultant glass laser rod, and (6) discharging the molten refined glassthrough the discharge opening for the production of high quality glasslaser rods having a minimum of undesirable platinum inclusions.

As seen in the drawings, the present invention comprises an apparatusfor melting and refining a lithium silicate glass composition having arelatively high amount of Li O in the range of at least about 8 molepercent, the apparatus comprising a furnace 1 having a bottom 3,sidewalls 4 and a roof 6 having a charge opening 7 for the charging ofnonvolatile or low volatile raw batch materials into the interior of thefurnace. As seen in the drawings, the sidewalls and bottom of thefurnace, as well as the roof, are provided with a liner 10 having aplatinum surface to hold the molten glass during the melting and finingthereof, the liner covering substantially the bottom, roof and sidewallsof the furnace which are exposed to the molten glass and the furnaceatmosphere. A stirrer 12 is provided with a platinum surface 13 forstirring a molten glass.

Means for introducing an inert gas such as nitrogen to the furnaceatmosphere over the molten glass is provided. The inert gas atmospheremeans includes an inlet conduit 16, a control valve 17 in the conduit16, an outlet conduit 18, and a source of inert gas under pressure, notshown. As best seen in FIG. 1, a solenoid valve 20 is provided in theoutlet conduit for shutting off the exhausting of the inert gas from thefurnace through the conduit 18 during the charging operation. The flowof inert gas such as nitrogen purges the furnace atmosphere duringcharging by escaping through the charge opening to prevent any air fromentering the furnace atmosphere during charging. The solenoid valve 20is then opened after the recharging period and the flow of gas continuedinto the furnace atmosphere from the inlet conduit and outlet conduitduring the melting, fining and discharging operations.

Means for withdrawing the molten glass from the furnace after it hasbeen thoroughly melted and fined includes a discharge opening 22 and aplunger 24 for opening and closing the discharge opening. Means forcharging the raw batch materials is provided, such means including thecharge opening 7 in the roof 6 and a charge tube 26 adapted to fitthrough the charge opening during the charging operation. A chargeopening cover 30 is provided to help block and seal the charge openingduring the melting and fining operation.

As previously indicated and as shown in the drawings, molten glasssealing means is provided for the charge means, the stirring means, theplunger and for the juncture between the roof and sidewalls so as toprevent condensation of vapors on colder areas and to prevent air fromentering the furnace atmosphere which could result in platinuminclusions in the molten glass and in the glass laser rod. With respectto the sealing of the above-described structures to the passage of airor platinum vapors, etc., it is preferred that the sealing means bemolten glass that usually is of a sodium boroaluminosilicate glasscomposition or other relatively viscous glass having a viscosity of atleast about log 2.5 or log 3 up to as high as log 4 or more at themelting temperature (generally about 2,300 F. to 3,000 F. and preferablyabout 2,500 to 2,700 F.) and being capable of keeping platinum vaporsfrom going through the seal from the inside of the furnace and keepingair from going through the seal from the outside to the inside of thefurnace. The preferred sodium boroaluminosilicate glass has acomposition containing magnesium oxide, calcium oxide and barium oxide.One highly preferred glass composition is one that contains thefollowing ingredients in approximate percentages by weight: SiO 66-69, BO 0.5-3, Al O 1.5-4, K 0.1-1.5, Na O 14-17, CaO 4-7, MgO 3-5, BaO 1-3.

In connection with the molten glass seal for the charge opening, thepreferred glass composition is less viscous than the previouslydescribed glass and generally has a viscosity of about log 1.5 to log2.2 and preferably about log 2 at the melting temperature. Onecomposition that has been used advantageously with the charge openingseal is one having a viscosity of about log 2 at the melting temperatureand having the following ingredients in approximate mole percentages:

Ingredients Percent SiO, 45-75 Li,0 45-35 C110 -30 A1 0 0-8 Anoutstanding glass composition for the charging means seal is onecomprising the following ingredients in approximate mole percentages:

Ingredients Percent SiO 60 Li,0 27.5 10 A1 0 2.5 Nd,0, 0.5

Turning to the sealing means between the juncture of the sidewalls andthe furnace roof, a roof baffle 32 is provided between the top of thesidewalls and the roof at the juncture thereof. A hollow sealingcontainer or member 34 located at the top of the sidewalls and the roofis provided and the container 34 is at least partially filled withmolten glass 36. As seen in the drawings, the roof baffle 32 extends atleast part way into the molten glass 36 to thereby prevent air fromentering the furnace atmosphere. The molten glass bath 36 also preventsthe passage of platinum vapors to colder surfaces and hence prevents thedeposit of platinum containing material in the molten glass which couldresult in platinum inclusions in the final product-the glass laser rod.

As previously indicated, sealing means is preferred for the stirrermeans, the stirrer means comprising a stirrer 12 having a rotatableshaft 39 and a blade 40. Container 42 is provided around the stirrershaft 39 which is located in the atmosphere above the molten glass nearthe roof 6. The stirrer shaft 39 passes through the container and thecontainer is filled with molten glass 46. There is provided a stirrerbaffle member 52 concentric to and outside the shaft 39. The end of thestirrer baffle member 52 extending into the molten glass 46 to therebyprevent air from entering the melting chamber and to prevent the depositof platinum vapors on colder areas such as the shaft and therebyeliminate platinum inclusions in the final product.

As previously described, there is provided a sealing means for theplunger, the plunger 24 having a shaft 54 passing through a hollowcontainer 55 which is filled at least part way with molten glass 56. Aspreviously indicated, there is provided a sealing means for the chargeopening 7, comprising the charge tube 26 preferably having a stainlesssteel surface 58 on at least a portion thereof exposed to the furnaceatmosphere during charging. Generally, the charge tube is exposed to theheat of the furnace only about 20 to 40 seconds. However, a stainlesssteel charge tube has been preferred to one of platinum. The preferredstainless steels including heat resistant steels containing generallyfrom about 65 to 95 percent by weight of iron and alloying metals suchas Cr, Mn, Ni and Co in amounts of about 1 or 2 up to 5 or 20 or morepercent.

The sealing means for the charge opening comprises a hollow circularmember 62 for sealing the charge tube opening. The circular member 62 islocated in the roof near the bottom thereof and around the outerperiphery of the charge opening 7. The container 62 holds molten glass66 which preferably has a relatively low viscosity of about log 2 at themelting temperature. The charge cover has a bottom portion 68 in theform of a baffle which extends into the circular container member 62 toprevent air from entering the furnace atmosphere from the outside andprevent the deposition of platinum vapors on cooler surfaces which inturn would tend to cause platinum inclusions in the final product. Thebottom of the charge cover and the baffle is preferably made of platinumor has a platinum surface. Likewise, the baffles for the juncturebetween the sidewalls and the roof and for the plunger and for thestirrer means are preferably made of platinum or have platinum surfaces.Although the preferred material is platinum for lining the furnaceinterior and for covering surfaces, etc. exposed to the furnaceatmosphere, in some cases where not as high a quality is desired, analloy of platinum/rhodium can be used in which the platinum content isto or 98 percent by weight of the alloy. In some cases other noblemetals such as silver and gold can be used in place of the platinum.

As previously indicated, the preferred gas is nitrogen although otherinert gases such as argon, helium, krypton and neon can be used for someapplications.

The use of the apparatus and the methods of the present inventionprovide for the production of high quality glass laser rods from anormally corrosive molten glass material. Outstanding glass rods can bemade of a normally corrosive composition at the melting temperaturewhich comprises the following ingredients in approximate molepercentages:

Ingredients Percent SiO, 45-75 Li O 15-35 CaO l t-30 Nd O 1/10-2 A1 0,,0-8

The above-described glass laser composition also preferably includes asolarization inhibiting oxide that is preferably cerium oxide in amountsof about 0.1 to 0.3 mole percent. The resulting glass laser composition,when made using the apparatus and methods of the present invention,results in a glass laser rod having a relatively high efficiency and lowsolarization in high power pulsed application, the glass laser rodhaving a minimum of undesirable platinum inclusions. Glass laser rodcompositions such as above-described are described in U.S. Pat. No.3,471,409 for an invention of Lee and Rapp for Glass Lasers For HighEnergy Application, which patent is hereby incorporated by reference.

Another patent, namely U.S. Pat. No. 3,457,182 for an invention of Leeand Rapp entitled Glass Compositions For Laser Application sets forthglass laser compositions suitable for use in the present invention. Thecompositions of this patent comprise the following ingredients inapproximate weight and mole percentages:

Ingredients Weight Percent Mole Percent SiO at least about 45 up to 8045-75 Al,0 about 4 up to 40 2-25 Li O about 5.5 up to about 20 8-40 Nd Oat least about 1 up to 8 0.2-2

The compositions of the above-identified patent make outstanding laserrods when processed according to the methods of the present inventionand using the apparatus thereof. The disclosure of the U.S. Pat. No.3,457,182 is also hereby incorporated by reference.

It has been found that a propeller stirrer generally produces the mosthomogeneously melted and refined molten glass. Although helical stirrersand stirrers of other design can be used which tend to mix and stir themolten glass well, the

propeller stirrer appears to shear, or break up the molten glass in afashion that leads to more homogeneous high quality glass.

As previously indicated, it is highly preferred the raw batch materialsbe introduced in a nonvolatile form in which preferably only oxides areused. Hence, for a glass containing SiO Li O, CaO, A1 0 Nd O pnly theoxides of these materials would be used to help reduce the corrosivenature of the melting of the raw batch materials. In this way, a batchof, say, 50.1 pounds can be used to prepare a glass with an equivalentweight of 50 pounds. If nonvolatile materials such as lithium carbonatewere to be used, the batch weight required would be much higher and thevolume would be larger. Some of the oxides can be combined as in thecase of lithium silicate which contains only U 0 and SiO By followingthe method steps previously discussed and using the apparatus describedherein, excellent high quality laser glass can be obtained with aminimum of platinum inclusions in spite of the difficulty of workingwith normally corrosive molten glass at high temperatures withconsiderable vibration due to the stirrers and plunger, etc.

What is claimed is:

1. An apparatus for melting and refining a silicate glass composition,the apparatus comprising:

a furnace having a bottom and sidewalls for holding the silicate glasscomposition,

a roof for the top of said furnace,

a liner having a platinum surface for the bottom and sidewalls of thefurnace to hold the molten glass,

a stirrer with a platinum surface for stirring the molten glass,

means for introducing an inert gas for the furnace atmosphere over themolten glass,

means for withdrawing the inert gas from the interior of the furnace,

means for withdrawing the molten glass from the furnace including adischarge opening and a plunger for opening and closing the dischargeopening,

means for charging raw batch materials into the furnace including acharge opening in the roof and a cover therefor that fits in the chargeopening, and

sealing means for the charge means, the stirring means, the

discharge means and the juncture between the roof and the sidewallscomprising molten glass seals to eliminate the condensation of platinumvapors on colder areas which could cause platinum inclusions in themolten glass, there being an air-tight glass seal to prevent travel ofair therethrough and positioned between the molten glass and openingsout of the furnace around each of the charge opening, the stirringmeans, the discharge means and juncture, each seal being so constructedand arranged that air from outside the furnace cannot travel into thefurnace atmosphere.

2. An apparatus as defined in claim 1 in which the means for introducingthe inert gas comprises an outlet conduit and a valve located thereinwhereby the flow of gas through the outlet conduit can be stopped whenthe furnace is being charged to purge out ambient air through the chargeopening and opened during the period of melting and refining the glass.

3. An apparatus as defined in claim 2 in which the molten glass sealscomprise a sodium boroaluminosilicate glass having a viscosity of aboutlog 2.5 to log 3 at the melting tempera tures, the molten glass keepingair from going therethrough.

4. An apparatus as defined in claim 2 in which the molten glass sealscomprise a sodium boroaluminosilicate glass composition containingmagnesium oxide, calcium oxide and barium oxide.

5. An apparatus as defined in claim 3 in which a roof baffle is providedbetween the top of the sidewalls and the roof at the juncture thereof,there being a hollow sealing member at least partially filled with themolten glass between the top of the sidewalls and the roof, the roofbaffle extending at least part way into the molten glass to therebyprevent air from entering the melting chamber and to minimize thecontamination of the molten laser glass in the furnace with platinumcontaining material that could cause platinum inclusions.

6. An apparatus as defined in claim 5 in which the stirrer has arotatable shaft and a blade, a hollow container is provided around thestirrer shaft located in the atmosphere above the molten glass and nearthe roof, the stirrer shaft going through the container and thecontainer filled with molten glass, there being provided a stirrerbaffle member concentric to and outside the shaft, the end of thestirrer baffle member extending into the molten glass, there being ahollow container around the plunger shaft in the furnace atmosphere, theplunger shaft passing through the container, the container being filledat least part way with molten glass, a concentric baffle outside theplunger shaft coming down from the roof and into the molten glass toprevent air from entering the melting chamber and to prevent deposits ofthe platinum containing materials on the stirrer shaft in the furnaceand on the plunger shaft to eliminate platinum inclusions.

7. An apparatus as defined in claim 1 in which the charge means includesa tube that is moved into the charge opening during charging, the tubehaving a stainless steel surface that is exposed to the furnaceatmosphere during charging.

8. An apparatus as defined in claim 6 in which the stirrer is apropeller stirrer.

9. An apparatus as defined in claim 2 in which the valve is a solenoidvalve.

10. A method of melting and refining molten glass for the production ofglass laser rods, the glass having a relatively high lithia content ofat least about 8 mole percent of the total glass composition, the glassbeing melted in a platinum-lined furnace having a roof, sidewalls and abottom having a discharge opening and a plunger that opens and closesthe opening, the method comprising the steps of l. charging into theplatinum-lined furnace by means of a charge opening batch materials in anonvolatile form,

II. melting the batch materials to form molten glass and refining thesame in a furnace,

III. providing an atmosphere of inert gas above the molten glass duringsteps 1 and II,

IV. stirring the molten glass with stirrers having platinum surfaces onat least a portion thereof exposed to the molten glass in furnaceatmosphere,

V. sealing openings from the furnace atmosphere to outside the furnacearound the charge opening, the stirrers, the juncture between thefurnace roof and sidewalls, and the plunger with molten glass seals toprevent air from enter ing the furnace atmosphere and to eliminatedeposits of platinum containing material which could cause platinuminclusions, there being a glass seal positioned between the molten glassand openings out of the furnace around each of the charge opening, thestirrers, the plunger and juncture, each seal being so constructed andarranged that vapors from the furnace atmosphere cannot travel throughthe seals and air from outside the furnace cannot travel into thefurnace atmosphere, and

VI. discharging the molten refined glass through the discharge openingfor the production of high quality glass laser rods having a minimum ofplatinum inclusions.

11. A method as defined in claim 10 in which, during the step III ofproviding an atmosphere of inert gas above the molten gas, includingstopping the flow of inert gas in an outlet conduit to prevent air fromentering the furnace atmosphere.

12. A method as defined in claim 11 in which the molten glass sealscomprise a sodium boroaluminosilicate glass having a viscosity of aboutlog 2.5 to log 3 at the melting temperatures, the glass seals keepingplatinum vapors from going through the molten glass and keeping air fromgoing through the molten glass.

13. A method as defined in claim 12 in which the molten glass sealscomprise a sodium boroaluminosilicate glass composition containingmagnesium oxide, calcium oxide and barium oxide.

14. A method as defined in claim 13 in which the glass laser compositioncomprises the following ingredients in the approximate mole percentages:

Ingredients SiO CaO Nd O CeO Percent 5-; 33 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 5, 5 ,9 Dated April l8, 1972Inventofle) George .C.v Chapman and James T. LeSueur It is certifiedthat error appears in the above-identified patentand that said LettersPatent are hereby corrected 'as shown below:

001.2, line 74, insert after 2700". Col. 5, line 20, "45-55" should be--l +-55--. Col.'- 4, line +8, 'applicalsion" should be --a.pp-licationsCol. 7, Claim 14, the chart containing the Ingredients and'Percentshouldread as follows:-

ling grwrd'mnts Percent mg; i M545 vL'l O 15-55 .0 0 1/2 5 M 0 1/10-2 IA1 0 a 0 8 CeOg 0-0,}

Signed and Sealed this 30th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETGHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. An apparatus as defined in claim 1 in which the means for introducingthe inert gas comprises an outlet conduit and a valve located thereinwhereby the flow of gas through the outlet conduit can be stopped whenthe furnace is being charged to purge out ambient air through the chargeopening and opened during the period of melting and refining the glass.3. An apparatus as defined in claim 2 in which the molten glass sealscomprise a sodium boroaluminosilicate glass having a viscosity of aboutlog 2.5 to log 3 at the melting temperatures, the molten glass keepingair from going therethrough.
 4. An apparatus as defined in claim 2 inwhich the molten glass seals comprise a sodium boroaluminosilicate glasscomposition containing magnesium oxide, calcium oxide and barium oxide.5. An apparatus as defined in claim 3 in which a roof baffle is providedbetween the top of the sidewalls and the roof at the juncture thereof,there being a hollow sealing member at least partially filled with themolten glass between the top of the sidewalls and the roof, the roofbaffle extending at least part way into the molten glass to therebyprevent air from entering the melting chamber and to minimize thecontamination of the molten laser glass in the furnace with platinumcontaining material that could cause platinum inclusions.
 6. Anapparatus as defined in claim 5 in which the stirrer has a rotatableshaft and a blade, a hollow container is provided around the stirrershaft located in the atmosphere above the molten glass and near theroof, the stirrer shaft going through the container and the containerfilled with molten glass, there being provided a stirrer baffle memberconcentric to and outside the shaft, the end of the stirrer bafflemember extending into the molten glass, there being a hollow containeraround the plunger shaft in the furnace atmosphere, the plunger shaftpassing through the container, the container being filled at least partway with molten glass, a concentric baffle outside the plunger shaftcoming down from the roof and into the molten glass to prevent air fromentering the melting chamber and to prevent deposits of the platinumcontaining materials on the stirrer shaft in the furnace and on theplunger shaft to elimiNate platinum inclusions.
 7. An apparatus asdefined in claim 1 in which the charge means includes a tube that ismoved into the charge opening during charging, the tube having astainless steel surface that is exposed to the furnace atmosphere duringcharging.
 8. An apparatus as defined in claim 6 in which the stirrer isa propeller stirrer.
 9. An apparatus as defined in claim 2 in which thevalve is a solenoid valve.
 10. A method of melting and refining moltenglass for the production of glass laser rods, the glass having arelatively high lithia content of at least about 8 mole percent of thetotal glass composition, the glass being melted in a platinum-linedfurnace having a roof, sidewalls and a bottom having a discharge openingand a plunger that opens and closes the opening, the method comprisingthe steps of I. charging into the platinum-lined furnace by means of acharge opening batch materials in a nonvolatile form, II. melting thebatch materials to form molten glass and refining the same in a furnace,III. providing an atmosphere of inert gas above the molten glass duringsteps I and II, IV. stirring the molten glass with stirrers havingplatinum surfaces on at least a portion thereof exposed to the moltenglass in furnace atmosphere, V. sealing openings from the furnaceatmosphere to outside the furnace around the charge opening, thestirrers, the juncture between the furnace roof and sidewalls, and theplunger with molten glass seals to prevent air from entering the furnaceatmosphere and to eliminate deposits of platinum containing materialwhich could cause platinum inclusions, there being a glass sealpositioned between the molten glass and openings out of the furnacearound each of the charge opening, the stirrers, the plunger andjuncture, each seal being so constructed and arranged that vapors fromthe furnace atmosphere cannot travel through the seals and air fromoutside the furnace cannot travel into the furnace atmosphere, and VI.discharging the molten refined glass through the discharge opening forthe production of high quality glass laser rods having a minimum ofplatinum inclusions.
 11. A method as defined in claim 10 in which,during the step III of providing an atmosphere of inert gas above themolten gas, including stopping the flow of inert gas in an outletconduit to prevent air from entering the furnace atmosphere.
 12. Amethod as defined in claim 11 in which the molten glass seals comprise asodium boroaluminosilicate glass having a viscosity of about log 2.5 tolog 3 at the melting temperatures, the glass seals keeping platinumvapors from going through the molten glass and keeping air from goingthrough the molten glass.
 13. A method as defined in claim 12 in whichthe molten glass seals comprise a sodium boroaluminosilicate glasscomposition containing magnesium oxide, calcium oxide and barium oxide.14. A method as defined in claim 13 in which the glass laser compositioncomprises the following ingredients in the approximate mole percentages:Ingredients Percent SiO245-75 Li2O 15-35 CaO 1/2-30 Nd2O31/10-2 Al2O30-8CeO20-0.3
 15. A method as defined in claim 12 in which the inert gas isnitrogen.
 16. A method as defined in claim 10 in which the raw batchmaterials are oxides of silica, lithium, calcium, aluminum andneodymium.
 17. A method as defined in claim 10 which includes the stepof purging the atmosphere above the molten glass during charging by aflow of inert gas into the furnace to force any air in the furnacethrough the charge opening.